Pearls, also known as oyster gems, are a type of gemstone formed inside the shells of certain mollusks. They are created when an irritant, such as a grain of sand, enters the mollusk's shell and the mollusk secretes a substance called nacre to coat the irritant. Over time, this process forms a pearl. Pearls come in a variety of shapes, sizes, and colors and are highly valued for their beauty and rarity.